1. Field of the Invention
The present invention relates to an exhaust gas recirculation sensor for controlling an amount of the recycling of exhaust gas from an automobile. The exhaust gas recirculation will be hereinafter referred to as EGR.
2. Description of the Prior Art
A conventional EGR sensor will be described with reference to FIGS. 12 to 14. A case 31 consisting of a mold of synthetic resin has a hollow housing portion 31a with its one side opened, a pair of concave shoulder portions 31b extending in the front to rear direction in the housing portion 31a, a pair of right and left guide portions 31c located above the shoulder portions 31b and extending in the front to rear direction in the housing portion 31a, and a tubular portion 31d protruding in the direction normal to the housing portion 31a. 
An insulating substrate 33 provided with a plurality of metal terminals 32 is mounted with a hole IC 34 connected electrically to the terminals 32. The insulating substrate 33 is inserted into a pair of the shoulder portions 31b of the case 31 to be mounted in the housing portion 31a. 
The terminals 32 mounted onto the insulating substrate 33 are soldered and connected to one end of an L-shaped pin 35 for connector embedded into the case 31.
A cover 36 consisting of a mold of synthetic resin has a bearing portion 36b provided in its center portion with a circular hole 36a and a convex portion 36c provided in the rear side portion of the outer circumference thereof. The cover 36 is mounted by fitting the convex portion 36c into the case 31 so as to cover the opening portion of the case 31.
An operating shaft 37 has a cylindrical shaft portion 37a, a stopper portion 37b provided at the rear side of the shaft portion 37a, and a convex portion 37c provided in the rear end of an axial direction G2. The shaft portion 37a is guided through the hole 36a of the cover 36 so as to move the operating shaft 37 linearly. The stopper portion 37b is contacted onto a rear end 36d of the bearing portion 36b so as to prevent the operating shaft 37 from coming off the bearing portion 36b. 
To assemble the operating shaft 37, the shaft portion 37a is first inserted from the rear side of the cover 36 through the hole 36a. Then, the stopper portion 37b is retained to the rear end 36d of the cover 36, and the operating shaft 37 is combined with the cover 36. Thereafter, while both are combined with each other, the cover 36 is assembled into the case 31.
A movable member 38 consisting of a mold of synthetic resin has a rectangular base portion 38a located in the center, a pair of spring receiving portion 38b extending from the base portion 38a to right and left and each having in its end a cross portion, and four sliding portions 38c each provided at the right or left side in the front or rear position of the base portion 38a. 
In addition, the movable member 38 is provided in its lower portion with a magnet 39. The movable member 38 provided with the magnet 39 is mounted onto the case 31 so as to be moved linearly while the magnet 39 is opposite to the hole IC 34 and the sliding portions 38c are located in the guide portion 31c of the case 31.
The magnet 39 and the hole IC 34 construct detection means K2.
Two coil springs 40 are disposed in the housing portion 31a so that one end of each of the coil springs 40 is contacted onto a back surface wall 31e, and the other end is contacted onto the spring receiving portion 38b of the movable member 38.
The two coil springs 40 bias the movable member 38 forward. The center portion of the movable member 38 is thus contacted resiliently onto the convex portion 37c of the operating shaft 37. The stopper portion 37b is contacted onto the rear end 36d of the bearing portion 36b to prevent the operating shaft 37 from coming off the bearing portion 36b. 
The operation of the conventional EGR sensor will be described. A rod for driving the EGR valve of an automobile pushes the operating shaft 37 against the coil springs 40. The operating shaft 37 allows the movable member 38 to be guided by a guide portion 31c and to move linearly in the axial direction G2.
The magnet 39 is also moved to fluctuate the magnetic field to the hole IC 34. As a result, the detection means K2 varies the electric current value so as to detect an amount of the operating shaft 37 moved.
When the pushing of the operating shaft 37 by the rod is released, the coil springs 40 push back the movable member 38, and the operating shaft 37 is also pushed back into the original state by the movable member 38.
Thus, the conventional EGR sensor can be operated.
In the convention EGR sensor, while the operating shaft 37 is assembled into the bearing portion 36b of the cover 36, the cover 36 is mounted onto the case 31. The assembling operation is troublesome, so that the productivity is poor, and the cost is high.
Further, in the EGR sensor, in the case where the operating shaft 37 is found to be scratched in quality inspection before shipping, the operating shaft 37 must be replaced. The cover 36 must be removed during the replacement. The replacement operation is troublesome, so that, substantially, the EGR sensor must be discarded, and parts other than the operating shaft 37 are wasted.
Accordingly, an object of the present invention is to provide an EGR sensor which facilitates the assembling operation and can easily replace the operating shaft.
As a first solving means for solving the foregoing problems, an exhaust gas recirculation sensor comprises an operating shaft movable linearly in the axial direction, a bearing portion for guiding movement of the operating shaft, a movable member movable linearly by the operating shaft, a spring member for returning movement of the movable member and biasing the movable member so as to resiliently contact the movable member onto the operating shaft, a case for guiding movement of the movable member, and detection means operated by movement of the movable member, wherein the operating shaft is provided in its rear portion with a convex portion protruding from a shaft portion in the direction orthogonal to the axial direction, the bearing portion is provided with a concave portion connected to a hole through which the shaft portion is inserted and allowing the convex portion to be inserted therethrough in the axial direction, and the convex portion protruding from the rear end of the bearing portion is retained to the rear end of the bearing portion other than the concave portion.
As a second solving means, the rear end of the bearing portion is provided with blocking portions for blocking movement of the convex portion to the concave portion.
As a third solving means, the blocking portions are formed by protrusions provided at both sides of the concave portion.